This invention relates in general to semiconductor processing techniques and in particular, to a technique for plating conductive bumps on the bond pads of one or more integrated circuit dies, and an apparatus optionally used in conjunction with the technique for holding a plurality of good integrated circuit dies while plating conductive bumps on their bond pads.
The formation of conductive bumps on the bond pads of an integrated circuit ("IC") die is especially useful in conjunction with Tape Automated Bonding ("TAB"). TAB is an effective and established means of reliably forming high density packaging. The TAB method uses a thermal bonding technique to bond inner leads formed on a film carrier to bumps typically formed on the bond pads of an IC die.
A critical process and a key element in the successful use of TAB is the proper formation of the bumps which serve to not only electrically connect the film carrier leads to the IC bond pads, but also to mechanically connect them. Several wafer processing techniques have been developed for forming bumps on the bond pads of dies preformed on a wafer, but each of these techniques has certain drawbacks. See, e.g., Liu, T. S. et al., "A Review of Wafer Bumping for Tape Automated Bonding," Solid State Technology, March 1980.
For example, one technique (hereinafter referred to as the "contact metal mask technique") for forming conductive bumps on the IC bond pads of dies on a wafer includes the steps of: placing a contact metal mask over the wafer being processed in such a fashion that holes in the metal mask align with IC bond pads of dies on the wafer; evaporating a layer of conductive material into the holes as well as over the exposed surface of the metal mask; and then removing the contact metal mask, leaving behind the thus formed conductive bumps on the IC bond pads of dies on the wafer. Although this technique comprises relatively simple processing steps, two problems with this technique are the added expense caused by either recovering the evaporated conductive material from the contact metal mask or wasting the conductive material by leaving it on the metal mask, and the difficulty in achieving a uniform height for the conductive bumps using the evaporation process.
Another technique (hereinafter referred to as the "photomask technique") for forming conductive bumps on the IC bond pads of dies on a wafer includes the steps of: depositing a layer of photoresist material over the active surface of the wafer; positioning an appropriately patterned photomask over the photoresist layer; exposing portions of the photoresist layer to an ultraviolet light source through portions of the photomask; removing the photomask and etching away portions of the photoresist layer deposited over the IC bond pads of the dies on the wafer; electroplating conductive material through holes formed by the etched portions of the photoresist layer; and removing the remaining photoresist layer, leaving behind the thus formed conductive bumps on the IC bond pads of the dies on the wafer. Although this technique overcomes the disadvantages of the contact metal mask technique, it adds additional processing steps through the use of a photoresist layer in forming the conductive bumps.
In addition, although wafer processing often provides certain economies of scale through the batch processing of dies, wafer processing of bumps can be inordinately expensive and its cost can exceed those economies of scale when gold ("Au") or other expensive, high purity material is used as the bump material. This is because wafer processing indiscriminately forms gold bumps on the bad dies, as well as the good dies of the wafer. The processing of large ICs is especially expensive, because of the smaller number of gross dies and the smaller percentage of good dies on the wafer being processed.
TAB, however, was developed as a packaging and mounting technique for handling large ICs with a large number of input and output ("I/O") bond pads. Thus, as ICs get larger, making TAB more attractive, the wafer processing cost of placing gold or other expensive, high purity material bumps on the IC bond pads rises exponentially with yield loss.
Alternative approaches, other than processing the bumps on the IC wafer, have therefore been pursued. One such approach is to eliminate the requirement of bumping the IC in favor of bumping the leads on the film carrier. See, e.g., Hatada, Kenzo et al., "Bump Property for High Bondability and Reliability in Transferred Bump Tab Assembly Technology," IMC 1988 Proceedings, Tokyo, May 25-27, 1988.
Bumping the film carrier leads, however, can also be unduly expensive for high volume production requirements since only one set of leads, corresponding to only one IC, can be fabricated at a time. Thus, although this technique solves the problem of wasting gold or other expensive, high purity bump material on bad dies, it loses the cost advantages resulting from the economies of scale provided by a batch processing fabrication method.